1. Field of the Invention
This invention relates to a microwave amplifier and more particularly to a broad-band microwave amplifier including a semiconductor diode operating in the TRAPATT mode and having a tapered idler circuit to provide a wide instantaneous bandwidth.
2. Description of the Prior Art
Microwave amplifiers having high power and high efficiency and wide bandwidths are desirable, for example, in phased-array radar systems, and communications systems. Prior art amplifiers utilizing TRAPATT diodes have generally required extensive circuit tuning to achieve a broad-band amplifier wherein the instantaneous bandwidth is made as wide as possible without sacrificing gain or efficiency. The instantaneous bandwidth of an amplifier, as known in the art, is the range of input frequencies over which a signal can be amplified at a particular instant of time without any mechanical or electrical adjustments or other tuning.
As is well known, the wave form of a signal generated or amplified by a semiconductor diode operating in the TRAPATT mode is rich in harmonic content. It is generally recognized that a successful TRAPATT amplifier design must provide certain load impedances at the fundamental trapped plasma frequency and at least the second and third harmonic thereof. The impedance loading conditions for such an amplifier are achieved in the prior art typically by using a separate tuned idler circuit for each frequency. For example, each circuit is tuned, i.e., adjusted to have a desired impedance condition, by placing an inductance element in series with a lumped capacitance element as shown in U.S. Pat. No. 3,909,740, issued on Sept. 30, 1975, or by placing tuning stubs in a transmission line as indicated in U.S. Pat. No. 3,848,196 issued on Nov. 12, 1974. In U.S. Pat. No. 3,868,588, issued on Feb. 25, 1975, one of the tuned circuits is eliminated by providing a semiconductor element which is self-resonant at one of the desired frequencies, thereby precluding the need for one of the circuits.
In addition to providing the appropriate impedance loading conditions at the harmonically related frequencies, the idler circuits are also utilized in some devices to provide for an increase in the instantaneous bandwidth. As shown in U.S. Pat. No. 3,852,680, which issued on Dec. 3, 1974, the idler circuits are comprised of rectangular-shaped microstrip transmission lines. Each transmission line has a selected characteristic impedance and a length of substantially one quarter wavelength of the harmonic frequency to which it is related. These distributed transmission lines present a complex impedance for terminating signals at certain harmonic frequencies. It has been found that the operating bandwidth is determined by a relatively small and controllable rate of change of the real and/or reactive portion of the complex impedance with respect to frequency. Such rectangular-shaped quarter wavelength microstrip idler circuits do not provide for as small a rate of impedance change as the present invention and thus result in a narrower instantaneous bandwidth.